It is obvious that the extruder alone is not sufficient to make extruded product. In addition to the extruder we need upstream and downstream equipment to produce a useful product.
The main elements of an extrusion line are:
• Resin handling system
• Drying system
• Extruder
• Post-shaping or calibrating device
• Cooling device
• Take-up device
• Cutter or saw
There are many different types of extrusion lines, the main types are:
• Tubing and pipe extrusion lines
• Film and sheet extrusion lines
• Extrusion compounding lines
• Profile extrusion lines
Besides these four main types there are quite a few more, such as fibre spinning lines, extrusion blowmoulding machines, integrated sheet and thermoforming lines, etc.
1.13.1 Tubing and Pipe Extrusion Lines
Dies for tubing and pipe were discussed earlier already. Small diameter tubing (less than about 10 mm) is usually made with a free extrusion process; this is a process without a sizing or calibrating unit. Large diameter tubing and pipe is made with a sizing device just downstream of the die.
The purpose of the sizing unit is to solidify the plastics in the calibrating section to a thickness sufficient to transfer the stresses acting on the product, while maintaining the desired shape and dimension. The main components of a typical tubing extrusion line are shown here.
This line does not use a sizing unit and, thus, would be used for small diameter tubing. The gear pump may or may not be used depending on the precision that is required in the extrusion process. The internal air pressure of the tubing is controlled to achieve the correct values for the outside diameter and wall thickness. The diameter is often measured with a laser gage to allow close monitoring and control of the diameter. The diameter and the wall thickness are determined mostly by the extruder output, the puller speed, and the internal air pressure. Closed loop control systems are available that automatically set the appropriate values screw or gear pump speed, the puller speed, and internal air pressure. After the puller the tubing may be cut or it may be reeled up on a spool. Tubing and pipe lines will be discussed in more detail in a later session.
1.13.2 Film and Sheet Lines Using the Roll Stack Process
There are no major differences between the extrusion of flat film and sheet.
The main components of a sheet line are the extruder, the roll stack, the cooling section, the nip roll section, and the winder (show figure). The roll stack contains three roll that are often referred to as polishing rolls. They are used to exert pressure on the sheet and to impart the surface conditions of the rolls to the plastics sheet. If a smooth surface is required, smooth rolls will be used. If a texture surface is needed, a textured surface is used on the roll. It is possible to have one textured surface and one smooth surface by having a smooth and textured roll next to each other.
The rolls are normally cored so that the temperature of the rolls can be controlled. This is usually done with circulation hot oil. The temperature of each roll can be adjusted separately. The rolls can be in a vertical position as shown or they can be at an angle. The cooling section consists of a number of roll positioned in a frame; the sheet is over and under the roll to keep the sheet flat.
At the end of the cooling section are the pull rolls or nip rolls; these are rubber rolls that pull the sheet from the roll stack to maintain a certain tension in the sheet. After the nip rolls, the sheet is led to the winder that rolls the sheet on a core. Many different winders are available; some winders automatically transfer the sheet to a new core when one package is full. Sheet lines will be covered in more detail in a later session.
1.13.3 Film Lines Using Chill Roll Casting
With thin film, the film is often cast on a chill roll rather than extruded into a roll stack. The main components of a cast film line are the extruder, the film die, the chill roll unit, the thickness gauging system, the surface treatment unit, and the winder.
The film is extruded downward onto the chill roll. The initial contact between the film and the chill roll is established by the use of an air knife. The air knife produces a thin stream of high velocity air across the width of the chill roll, the air stream pushes the film against the roll surface.
From the chill roll unit the film is lead to a thickness gauging unit where the thickness of the sheet is measured across the width of the film. Most thickness gages for film and sheet have a scanning measuring head that traverses the film back and forth to measure thickness both along the length and across the width of the film.
After the thickness gauging unit the film passes through a surface treatment unit. Such a unit is incorporated if a surface treatment of the film is required. This is usually done to improve adhesion, for instance for a subsequent printing or laminating operation.
The most important adhesion promoters are:
• Flame treatment
• Corona discharge treatment
• Ozone treatment
• Primers
From the treatment unit the film is led to the winder unit. Just as with sheet extrusion, many different types of winders are available. Cast film lines will be covered in more detail in a later session.
1.13.4 Combination of Materials
The requirements of many products, particularly in packaging applications, are such that they cannot be met by a single plastics. In order to meet the requirements often two or more materials have to be combined. There are a number of techniques to combine different materials; some of the more important ones are: co extrusion, coating, and lamination. We will discuss these in more detail.
